Light is one of the most important cues synchronizing an organism's internal Circadian Tmekeeping System (CTS) with the external environment. Desynchronization of the CTS may be responsible for many sleep and psychiatric disorders. Different colors of light have different efficacies in treating Seasonal Affective Disorder in humans. In mammals, light transmits temporal information to a neural clock in the Suprachiasmatic Nucleus. Few studies have tried to characterize the spectral and scotopic qualities of light important for entraining (synchronizing) the mammalian CTS. It is extremely difficult to extrapolate results obtained in rodents (with primarily rod photoreceptors) to human and nonhuman primates (with both rod and cone photoreceptors). It is hypothesized that the squirrel monkey CTS has differential sensitivity to light intensities and wavelengths. Our objective is to characterize the circadian responses of the rhythms of body temperature and activity in squirrel monkeys exposed to Light-dark (LD) cycles of varying durations, intensities, and wavelengths. Unrestrained adult male squirrel monkeys were surgically implanted with biotelemetry transmitters to record body temperature and activity. Subjects were isolated from extraneous environmental stimuli. The animals are first entrained to an LD cycle (e.g., LD 12:12) of a given wavelength. The light intensity was reduced by 50% approximately every two weeks until the animal is no longer entrained to the ambient LD cycle, but begins to live a day-length determined by its internal biological clock. The light wavelengths used ranged from blue to nearinfrared (400 - 1000 nM). Near infrared does not produce heat. We have shown that the minimum light intensity necessary to synchronize the CTS differs between l of light; for example, red light requires a higher intensity than blue light. We also demonstrated that squirrel monkeys, unlike humans, under non-24 hour LD cycles do not show internal desynchronization of rhythms. *KEY*Biological rhythms, Body temperature, Light, Eye